The present invention relates to method and apparatus using continuous suppression of electrolyte in eluents particularly for the analysis of anions or cations in ion chromatography.
Ion chromatography is a known technique for the analysis of ions which typically includes a chromatographic separation stage using an eluent containing an electrolyte, and an eluent suppression stage, followed by detection, typically by an electrical conductivity detector. In the chromatographic separation stage, ions of an injected sample are eluted through a separation column using an electrolyte as the eluent. In the suppression stage, electrical conductivity of the electrolyte is suppressed but not that of the separated ions so that the latter may be determined by a conductivity cell. This technique is described in detail in U.S. Pat. Nos. 3,897,213, 3,920,397, 3,925,019 and 3,926,559.
Suppression or stripping of the electrolyte is described in the above prior art references by a bed of ion exchange resin particles commonly referred to as a packed bed suppressor (PBS). The PBS requires periodic regeneration by flushing with an acid or base solution.
While packed bed suppressors have proven useful in ion chromatography, there are a number of disadvantages of a PBS. These disadvantages include a) periodic regeneration of the PBS which interrupts sample analysis, b) a loss of resolution due to band broadening in the PBS and c) changes in retention of certain analytes as a function of the degree of exhaustion of the PBS.
The volume and capacity of the PBS is generally large relative the separation column to contain sufficient ion exchange resin so that the suppression reaction can be performed for a large number of analysis (e.g. 15 to 50) prior to regeneration. By making the volume and capacity of the suppressor sufficiently large, the need to regenerate is less frequent which permits a larger number of samples to be analyzed before the system must be disrupted to regenerate the suppressor. Regeneration typically requires placing the suppressor out of line of the analytical system and pumping a concentrated acid or base solution (regenerant) through the suppressor.
If the suppressor's void volume is too large, the separation of the analytes achieved in the separator column is compromised due to re-mixing of the analytes in the void volume, resulting in lower resolution. Thus, the suppressor volume is a compromise between regeneration frequency and chromatographic resolution.
The regeneration process typically requires 20-60 minutes, depending on the volume of the suppressor. A strong acid or base solution is first pumped through the PBS in order to convert the resin to the acid (H.sub.3 O.sup.+) or base (OH.sup.-) form. After this conversion, deionized water is pumped through the suppressor until any traces of the highly conductive acid or base regenerant have been removed. The PBS is then placed back in line with the analytical system and is allowed to equilibrate before sample analysis is performed.
In U.S. Pat. Nos. 5,597,734 and 5,567,307, a method is described of regenerating a packed bed suppressor after each analysis. In this apparatus, the packed bed suppressor has limited capacity for just one or several sample analysis before the suppressor requires regeneration. The liquid flow through the low volume packed bed suppressor is used with suitable valving to pass liquid stream through the system. During analysis, eluent from the separator passes through the suppressor and to the conductivity cell. Immediately after the analysis, valving diverts a flow of chemical regenerant through the suppressor for regeneration. The valving then diverts eluent to the suppressor for equilibration prior to sample analysis. The regeneration and equilibration of this type of PBS can be performed in a short time with a small volume PBS.
Another form of packed bed suppression uses intermittent electrolytic regeneration as described and published in U.S. Pat. No. 5,633,171. A commercial product using this form of suppression is described in "Electrochemically regenerated solid-phase suppressor for ion chromatography" Saari-Nordhaus, R. and Anderson, J. M., American Laboratory, February 1996. In this product, an electrical potential is applied through the resin in the packed bed suppressor while flowing an aqueous liquid stream to electrolyze water in the stream. For the analysis of anions, a PBS containing fully sulfonated cation exchange is fitted with a cathode embedded in the resin at the suppressor inlet and an anode embedded in the resin at the suppressor outlet. Hydronium ions generated at the anode displace the sodium ions which associate with the hydroxide ions for passage to waste, in this instance through the conductivity cell. This process electrochemically regenerates the suppressor, and after the electrical potential is turned off, the device can be used as a conventional PBS. In a further embodiment, a second ion exchange resin bed is used with suitable valving to pass liquid streams through the system. In one alternative of this system, a second sample in an eluent stream is chromatographically separated, typically on a chromatographic column using an eluent. The eluent and separated second sample flow through a second packed bed suppressor including ion exchange resin to convert the electrolyte to weakly ionized form. Then, the separated sample ionic species in the suppressor effluent are detected in the detector. The effluent then flows through the first packed bed suppressor, forming the aqueous liquid stream required for regeneration and an electrical potential is applied and regeneration of the first packed bed suppressor is accomplished. The second suppressor may be similarly regenerated by positioning it after the detection cell and flowing through the detector effluent of the first sample and applying an electrical potential. This form of suppression does not require an external regenerant source and allows for uninterrupted operation although it is not considered continuous. This system uses two PBS's, additional valving and electronics to control the valve switching and timing.
A different form of a suppressor is described and published in U.S. Pat No. 4,474,664, in which a charged ion exchange membrane in the form of a fiber or sheet is used in place of the resin bed. The sample and eluent are passed on one side of the membrane with a flowing regenerant on the other side, the membrane partitioning the regenerant from the effluent of the chromatographic separation. The membrane passes ions of the same charge as the exchangeable ions of the membrane to convert the electrolyte of the eluent to weakly ionized form, followed by detection of the ions.
Another suppression system is disclosed in U.S. Pat. No. 4,459,357. There, the effluent from a chromatographic column is passed through an open flow channel defined by flat membranes on both sides of the channel. On the opposite sides of both membranes are open channels through which regenerant solution is passed. As with the fiber suppressor, the flat membranes pass ions of the same charge as the exchangeable ions of the membrane. An electric field is passed between electrodes on opposite sides of the effluent channel to increase the mobility of the ion exchange. One problem with this electrodialytic membrane suppressor system is that high voltages (50-500 volts DC) are used. As the liquid stream becomes deionized, electrical resistance increases, resulting in substantial heat production. Such heat can be detrimental to effective detection because it increases noise and decreases sensitivity.
In U.S. Pat. No. 4,403,039, another form of electrodialytic suppressor is disclosed in which the ion exchange membranes are in the form of concentric tubes. One of the electrodes is at the center of the innermost tube. One problem with this form of suppressor is limited exchange capacity. Although the electrical field enhances ion mobility, the device is still dependent on diffusion of ions in the bulk solution to the membrane.
Another form of suppressor is described in U.S. Pat. No. 4,999,098. In this apparatus, the suppressor includes at least one regenerant compartment and one chromatographic effluent compartment separated by an ion exchange membrane sheet. The sheet allows transmembrane passage of ions of the same charge as its exchangeable ions. Ion exchange screens are used in the regenerant and effluent compartments. Flow from the effluent compartment is directed to a detector, such as an electrical conductivity detector, for detecting the resolved ionic species. The screens provide ion exchange sites and serve to provide site to site transfer paths across the effluent flow channel so that suppression capacity is no longer limited by diffusion of ions in the bulk solution to the membrane. A sandwich suppressor is also disclosed including a second membrane sheet opposite to the first membrane sheet and defining a second regenerant compartment. Spaced electrodes are disclosed in communication with both regenerant chambers along the length of the suppressor. By applying an electrical potential across the electrodes, there is an increase in the suppression capacity of the device. The patent discloses a typical regenerant solution (acid or base) flowing in the regenerant flow channels and supplied from a regenerant delivery source. In a typical anion analysis system, sodium hydroxide is the electrolyte developing reagent and sulfuric acid is the regenerant The patent also discloses the possibility of using water to replace the regenerant solution in the electrodialytic mode.
Another improvement in suppression is described in U.S. Pat. No. 5,248,426. This form of suppressor was introduced in 1992 by Dionex Corporation under the name "Self Regenerating Suppressor" (SRS). A direct current power controller generates an electric field across two platinum electrodes to electrolyze water in the regenerant channels. Functionalized ion-exchange screens are present in the regenerant chambers to facilitate electric current passage with permselective ion-exchange membrane defining the chromatography eluent chamber, as in the '098 patent. After detection, the chromatography effluent is recycled through the suppressor to form a flowing sump for electrolyte ion as well as providing the water for the electrolysis generating acid or base for suppression. Thus, no external regenerant is required and the suppressor is continuously regenerated.
In copending application, Ser. No. 08/925,813, filed Sep. 4, 1997, now abandoned entitled Ion Chromatographic Method and Apparatus Using a Combined Suppressor and Eluent Generator, incorporated herein by reference ("the copending application"), method and apparatus are provided for generating an acid or base eluent in an aqueous solution and for simultaneously suppressing conductivity of the eluent in an ion exchange bed after chromatographic separation in an ion chromatography system. Referring first to the apparatus, the suppressor and eluent generator comprises: a flow-through suppressor and eluent generator bed of ion exchange resin having exchangeable ions of one charge, positive or negative, having an inlet and an outlet section in fluid communication with fluid inlet and outlet conduits, respectively; an electrode chamber disposed adjacent to said suppressor and eluent generator bed inlet section and having fluid inlet and outlet ports; a flowing aqueous liquid source in fluid communication with said electrode chamber inlet port; a first electrode disposed in said electrode chamber; a barrier separating said suppressor and eluent generator bed from said electrode chamber, the barrier preventing significant liquid flow but permitting transport of ions only of the same charge as said suppressor and eluent generator bed resin exchangeable ions; and a second electrode in electrical communication with said resin bed outlet section.
In one embodiment of the copending application ion chromatography apparatus, the generator is used with a flow-through separator bed of ion exchange resin having exchangeable ions of opposite charge to the exchangeable ions of said suppressor and eluent generator bed, said separator bed having a sample inlet port and an effluent outlet port, said electrode chamber outlet port being in fluid communication with said separator bed inlet port, said separator bed outlet being in fluid communication with said suppressor and eluent generator bed inlet port, and a detector downstream from the generator. The aqueous liquid source can be an independent reservoir or can be a recycle conduit from the detector.
For anion analysis, one method includes (a) flowing an aqueous liquid sample stream containing anions to be detected and cation hydroxide through a separator bed of anion exchange resin with exchangeable anions to form liquid effluent including separated anions and said cation hydroxide; (b) flowing said aqueous effluent from said separator bed through a flow-through suppressor and eluent generator bed comprising cation exchange resin including exchangeable hydronium ions, so that said cation hydroxide is converted to weakly ionized form, and some of said exchangeable hydronium ions are displaced by cations from said cation hydroxide, said suppressor and eluent generator bed having inlet and outlet sections and inlet and outlet ports, liquid effluent from said suppressor and eluent generator bed flowing through said outlet port; (c) flowing an aqueous liquid through a cathode chamber proximate to said suppressor and eluent generator bed inlet section and separated by a barrier therefrom, said barrier substantially preventing liquid flow between said cathode chamber and said suppressor and eluent generator bed inlet section while providing a cation transport bridge therebetween; (d) applying an electrical potential between a cathode in said cathode chamber and an anode in electrical communication with said suppressor and eluent generator bed outlet section, whereby water is electrolyzed at said anode to generate hydronium ions to cause cations on said cation exchange resin to electromigrate toward said barrier and to be transported across said barrier toward said cathode in said cathode chamber while water in said chamber is electrolyzed to generate hydroxide ions which combine with said transported cations to form cation hydroxide in said cathode chamber; (e) flowing said cation hydroxide from said cathode chamber to the inlet of said separator column; and (f) flowing the effluent liquid from said suppressor and eluent generator bed past a detector in which said separated anions are detected.
After passing the detector in step (f), the effluent liquid can be recycled to said cathode chamber. The system can be used for cation analysis by appropriate reversal of the cation and anion functional components.
In a second embodiment of the copending application suppressor and eluent generator bed, the second electrode is not in direct contact with the suppressor and eluent generator bed. Instead, it is adjacent the suppressor and eluent generator bed outlet section in a second electrode chamber similar to the one described above. In this embodiment, aqueous liquid exiting the detector may be recycled to the inlet of the second electrode chamber.
In a third embodiment, similar to the second one, aqueous liquid from a reservoir is pumped to the inlet of the second electrode chamber. Liquid from the outlet of the second electrode chamber is directed to the inlet of the first electrode chamber. Liquid flowing out of the first electrode chamber is directed to the inlet of the separator bed.
The copending application also discloses a method of anion analysis using two electrode chambers separated from the suppressor and eluent generator bed which includes the following steps: (a) flowing an aqueous liquid sample stream containing anions to be detected and a cation hydroxide through a separator bed of anion exchange resin with exchangeable anions to form a liquid effluent including separated anions and said cation hydroxide; (b) flowing said aqueous liquid effluent from said separator bed through a flow-through suppressor and eluent generator bed comprising cation exchange resin including exchangeable hydronium ions, so that said cation hydroxide is converted to weakly ionized form, and some of said exchangeable hydronium ions are displaced by cations from said cation hydroxide, said suppressor and eluent generator bed having inlet and outlet sections and inlet and outlet ports, liquid effluent from said suppressor and eluent generator bed flowing through said outlet port; (c) flowing an aqueous liquid through an anode chamber proximate to said suppressor and eluent generator bed outlet section and separated by a first barrier therefrom, said first barrier substantially preventing liquid flow between said anode chamber and said suppressor and eluent generator bed outlet section while providing a cation transport bridge therebetween, said aqueous liquid exiting said anode chamber as an anode chamber aqueous liquid effluent; (d) flowing an aqueous liquid through a cathode chamber proximate to said suppressor and eluent generator bed inlet section and separated by a second barrier therefrom, said second barrier substantially preventing liquid flow between said cathode chamber and said suppressor and eluent generator bed inlet section while providing a cation transport bridge therebetween; (e) applying an electrical potential between an anode in said anode chamber and a cathode in said cathode chamber, whereby water is electrolyzed at said anode to generate hydronium ions which are transported across said first barrier to cause cations on said cation exchange resin to electromigrate toward said second barrier and to be transported across said second barrier toward said cathode in said cathode chamber while water in said cathode chamber is electrolyzed to generate hydroxide ions which combine with said transported cations to form cation hydroxide in said cathode chamber; (f) flowing said cation hydroxide from said cathode chamber to the inlet of said separator bed; and (g) flowing the effluent from said suppressor and eluent generator bed past a detector in which said separated anions are detected.
The anode chamber aqueous liquid effluent may be recycled through said cathode chamber. Alternatively, after detection in step (g), the suppressor and eluent generator bed effluent may be recycled through said anode chamber.
The history of ion chromatography suppression as of 1993 was summarized in Rabin, S. et al. J. of Chromatog. 640 (1993) 97-109, incorporated herein by reference.